DC--DC converters are known to convert one DC (Direct Current) voltage to another DC voltage. For example, a particular DC--DC converter, in a commercial setting, may convert a 12 volt supply to a 5 volt supply or a 3 volt supply to a 5 volt supply.
The basic topology of a DC--DC converter may be of a variety of forms. For example, the topology of a DC--DC converter may be a full-bridge inverter, a half-bridge inverter, a buck converter, a boost converter, or a flyback converter. For low-power application, i.e. below 50 watts, the buck, boost, or flyback topologies are used, while for high-power applications, i.e. above 50 watts, the half-bridge or full-bridge inverters are used.
In integrated circuit (IC) technologies, the power requirements are relatively low, i.e. less than 1 watt. As such, a buck or boost topology is generally used for IC applications. A boost topology is used when the desired output DC voltage needs to be greater than the input voltage, while a buck converter is used when the output voltage needs to be less than the input voltage. Generally, a buck converter includes an inductor, a switching transistor, a diode, and a capacitor. When the switching transistor is conducting, energy is being stored in the inductor. When the switching transistor is turned off, the energy is transferred from the inductor through the diode to the capacitor. The output voltage (the voltage across the capacitor) is monitored by a feedback circuit which regulates the pulse-width of a control signal that gates the switching transistor. In this manner, the output voltage can be tightly regulated, typically within one percent of the desired output value.
As the consumer market drives technology to create portable devices that have extended battery life, many applications are utilizing low voltage. Thus, many ICs are being designed to operate off of low supply voltages, i.e. 2.7-3.3 volts. While most of the components within these ICs can operate at low voltages, there are a few components that require higher voltages to achieve proper performance. For these ICs, a boost DC--DC converter is required. In these applications, the switching transistor, the control circuit, and the diode of the boost converter may be on-chip, while the inductor and capacitor are off-chip.
As is known, many DC--DC converters include current-limiting circuits. One such current limiting circuit incorporates a resistor in series with the switching transistor to establish a current-referenced voltage. This voltage is compared with a reference voltage such that when it exceeds this reference voltage, the DC--DC converter is exceeding a predetermined maximum current. Another type of current-limiting circuit incorporates a current sensing transformer which senses the current through the switching transistor and provides a voltage representation thereof which is compared as previously mentioned. While these circuits adequately monitor current in the circuit, the addition of a current sensing resistor reduces the overall efficiency of the DC--DC converter due to power losses, and the current sense transformer adds both complexity and cost to the circuit. Note that for IC applications, the current-sense transformer would have to be off-chip, thus requiring additional external components for users of the IC.
Therefore, a need exists for a current limiting circuit that can be incorporated within an IC without the power losses of the above mentioned prior-art techniques and without the need for additional external components.